In order to increase the power reserve, by increasing the number of turns of a mainspring, secured at the inner end thereof to a core formed either by an arbour, which is generally cylindrical, or by a more solid hub, one solution consists in decreasing the diameter of the barrel arbour and of the associated hub, so as to increase the space available for the spring inside the drum.
The ratio of the core radius to the spring thickness is usually comprised between 10 and 20 and the invention proposes to reduce this ratio to below 10, and preferably to within a range of between 5 and 10.
The sizing must not be made rashly; there is a risk of breakage if the diameter of the core is too small.
In the conventional barrel architecture, a ratchet is axially mounted on a barrel arbour or on a core, via a square, with the ratchet usually being secured by an axial screw. The dimension of this screw and that of the square thus define the minimum diameter of a pivot shoulder. A step joined to this pivot shoulder limits the axial shake of the arbour or of the core relative to a bottom plate or to a bridge carrying a jewel or similar element.
In particular, it is not sufficient merely to reduce all of the dimensions, since the cross-sections of material are then insufficient to ensure fatigue resistance.
The issue is thus to reconcile the smallest possible diameter, to allow the largest possible power reserve, with rigidity in the ratchet drive.
U.S. Pat. No. 1,561,673 A in the name of Ulery, published in 1925, discloses a barrel with an arbour, and including a collar acting as support for a toothed wheel driving the drum. This arbour includes two housings at its opposite ends, one of which receives a pin for guiding the arbour in a bearing housed in a bottom plate and holding the end of said arbour in said bearing, and the other of which receives a pinned conical peg which has a similar shoulder for upper guiding in a bridge, and, at the distal end thereof, a ratchet drive square.
CH Patent No 15286 A in the name of Danasino, published in 1898, discloses a barrel with a smooth arbour which carries a hollow fusee including a ratchet. This hollow fusee has two shoulders receiving the drum and the cover. The cross-section of the mainspring increases gradually from the outer end towards the inner end thereof.
U.S. Pat. No. 168,581 A in the name of Potter, published in 1875, discloses a barrel with a substantially smooth arbour pivoting at both ends thereof between two plates on either side of a bottom plate in which the barrel is embedded and which forms a cover for the drum. This arbour carries a hollow fusee carrying a wheel and onto which the inner end of the spring is hooked. In abutment on the fusee, the arbour carries the drum onto which the outer end of the spring is hooked. Next, the arbour carries the ratchet, which is in abutment on a small step of the arbour and confined underneath one of the lateral plates.